Bipropellant gel propulsion systems spray oxidizer and fuel gel in contact with each other. These droplets then ignite hypergolically to produce the thrust for the gel propellant engine. These bipropellant gel systems have enough impulse for tactical applications but the density-impulse is lower than desired. The baseline fuel gel used by the Army is a 48% monomethylhydrazine, 1.5% hydroxypropyl cellulose and 50% carbon formulation. The oxidizer gel is inhibited red fuming nitric acid (IRFNA) type IIIB that is gelled with a 4.5% silicon dioxide. IR-FNA type IIIB is nitric acid that contains 14% nitrogen tetroxide. This combination of fuel and oxidizer has an optimum oxidizer to fuel ratio (O/F ratio) of 3.4.
Therefore, the largest improvement in density impulse occurs when modifications are made in the oxidizer gel.